Fullerenes are closed cage carbon compounds containing both six- and five-member carbon rings (See: Curl et al. (1991) Sci. Am. October, p. 54–63; Kratschmer et al. (1990) Nature 347:354–358; Diederich et al. (1991) Science 252:548–551). Fullerenes have a wide-range of potential commercial applications in fields ranging from use as pharmaceuticals and cosmetic additives to use as additives in electron- and photo-resists, proton-conducting membranes for fuel cells, optical limiting materials and devices, lithium battery anodes, active elements in organic transistors, and as pigments.
The term “carbon nanomaterials” is used generally herein to refer to any substantially carbon material containing six-membered rings that exhibits curving of the graphite planes, generally by including five-membered rings amongst the hexagons formed by the positions of the carbon atoms, and has at least one dimension on the order of nanometers. Examples of carbon nanomaterials include, but are not limited to, fullerenes, single-walled carbon nanotubes (SWNTs), multiple-walled carbon nanotubes (MWNTs), nanotubules, and nested carbon structures with dimensions on the order of nanometers. The term “fullerene” is used generally herein to refer to any closed cage carbon compound containing both six-and five-member carbon rings independent of size and is intended to include the abundant lower molecular weight C60 and C70 fullerenes, larger known fullerenes including C76, C78, C84 and higher molecular weight fullerenes C2Nwhere N is 50 or more. The term is intended to include “solvent extractable fullerenes” as that term is understood in the art (generally including the lower molecular weight fullerenes that are soluble in toluene or xylene) and to include higher molecular weight fullerenes that cannot be extracted, including giant fullerenes which can be at least as large as C400. Carbon nanomaterials may be produced in soot and, in certain cases, carbon nanomaterials may be isolated from the soot or enriched in the soot. Soot produced during the synthesis of carbon nanomaterials, such as fullerenes, typically contains a mixture of carbon nanomaterials which is a source for further purification or enrichment of carbon nanomaterials or which may itself exhibit desired properties of carbon nanomaterials and be useful as an addition to convey those properties. The term “carbon nanomaterials,” when used without limitation, is intended to include soot containing detectable amounts of carbon nanomaterials. For example, the term “fullerenic soot” is used in the art to refer to soot containing fullerenes. Fullerenic soot is encompassed by the term carbon nanomaterials.
At present, the cost of production of fullerenes is generally too high for their economic use in commercial applications. In view of the considerable body of potential applications of these materials, there is a significant need in the art for lower-cost methods for production of fullerenes and other carbon nanomaterials.
This invention relates generally to the development of low-cost methods for the production of fullerenes and other carbon nanomaterials and, in particular, focuses on the identification of low-cost carbon feedstocks which provide higher yields of such products or higher carbon conversion rates to such products.
Methods are known in the art for production of fullerenes in electric arcs (U.S. Pat. Nos. 5,227,038 and 5,876,684), by electric beam evaporation (U.S. Pat. No. 5,316,636) and combustion in flames (e.g., sooting flames) (U.S. Pat. Nos. 5,273,729; 5,985,232 and 6,162,411; Howard et al. (1991) Nature 352:139–141; Howard et al. (1992) J Phys. Chem. 96:6657; Howard et al. (1992) Carbon 30:1183; McKinnon et al. (1992) Comb. Flame 88:102).
Unsaturated hydrocarbon fuels, typically those containing benzene and acetylene, have been used for combustion synthesis of fullerenes. The fullerene yield from acetylene is very poor (Howard et al. (1992) Carbon 30:1183). The yield from benzene in a premixed flame at low pressure (ca. 35 Torr) under strongly sooting conditions is about 0.5% conversion of benzene carbon to carbon of solvent extractable fullerenes.
The detection of fullerenes on combustion of naphthalene was reported by Bachmann et al. (1994) Chem. Phys. Lett. 223:506–510. In this study, fullerene concentration was reported to be higher in certain parts of the naphthalene flame than in a benzene flame, but no yield increase was established because soot was not recovered from the naphthalene flame.
Taylor et al. (1993) Nature 366:728–731 reported the formation of C60 by pyrolysis of naphthalene at about 1,000° C. The fullerene yield from this process was reported to be<0.5% based on naphthalene consumed and was said to be “variable.” Further, fullerenes were not detected in naphthalene pyrolysis at lower temperatures (about 500° C.).
Zhang et al. (1999) J. Phys. Chem. B v. 103 p. 9450–9454 reports detection of fullerene ions by laser ablation mass spectrometry of pyrolyzed Koppers coal-tar pitch. The coal-tar pitch described as consisting mainly of polycyclic aromatic hydrocarbons (PAHs) was pyrolyzed under a helium flow for 1 h at temperatures between 200–600° C. to generate a solid. Fullerene ions (e.g., C60+) were detected when samples of the pyrolyzed solid residue were vaporized using laser ablation.
U.S. Pat. Nos. 5,985,232 and 6,162,411 both relate to the production of fullerenic soot in flames with emphasis on the production of “fullerenic nanostructures” or “fullerenic carbon sheets.” The fuel employed is termed “unsaturated hydrocarbons.” This term relates particularly to benzene and acetylene, but is also defined in the patents to include, without limitation, “ethylene, toluene, propylene, butylene, naphthalene or other polycyclic aromatic hydrocarbons such as, in particular, petroleum, heavy oil, and tar” as well as “products derived from coal, kerogen and biomass which are primarily hydrocarbon, but also contain some amounts of nitrogen, sulfur, oxygen and other elements.” These patents, however, do not teach or suggest that the use of naphthalene, any other polycyclic aromatic hydrocarbons, or any particular product of coal or petroleum will result in any significant improvement in fullerene production.